Palletized loads are often loaded and unloaded using a plurality of carriages that alternate between two positions on a continuous looped track, which may be either on a horizontal plane (like a race track) or in a vertical plane (like a ferris wheel). While both these layouts are relatively simple mechanically, they consume relatively large amount of floor space. Accordingly, a need has been apparent for apparatus for alternating a plurality of carriages between two positions that would consume less floor space, even at the cost of some increase in the complexity of the mechanisms involved.
An important solution to this problem was provided by the apparatus disclosed in my previous patent, U.S. Pat. No. 5,020,382, issued Jun. 4, 1991. Since that invention is related to the present invention, the disclosure of that patent is hereby incorporated herein by reference.
The invention disclosed in my previous patent interchanges the positions of two carriages. That is, if two carriages A and B start off in the positions X and Y, respectively, after one sequence of operations the carriage A has moved to the position Y and the carriage B has moved to the position X. Then, after another sequence of operations, the carriage A returns to the position X, and the carriage B returns the position Y. If the position X is, for instance, an off loading position, and the position Y is an on loading position, that is entirely satisfactory in many (probably most) situations.
However, it is sometimes desired to have both of the carriages alternately occupy a single position during a first stage of operations, but each carriage occupy a different, vertically spaced position at a second stage of operations. That is, if the two carriages A and B start off in the positions X and Y, respectively, after one sequence of operations the carriage A has moved to the position Y, and the carriage B has moved to the position X.sup.1, which is vertically spaced relative to the position X. Then, after another sequence of operations, the carriage A returns the position X, and the carriage B returns to the position Y. The positions X and X.sup.1 can be either off loading positions or on loading positions, depending on the desired use of the apparatus.